Devices for measuring the fill level of a fill material in a container by means of measurement signals guided along a conductive element are already known from the state of the art. As an example of a measurement apparatus, which determines the fill level using the travel time of high frequency measurement pulses (-TDR-method), the LEVELFLEX may be named, an apparatus marketed and sold by the applicant. Also known are FMCW-measurement apparatuses, which determine fill level on the basis of frequency modulated, continuous, electromagetic waves. A measurement apparatus and method, by which the fill level can be established, are known, for example from EP 1 020 735 A2.
Measurement apparatuses of the above-named type are usually secured to the container with a screw thread or a flange. During installation, it must be assured that the measurement apparatus is placed above the maximum fill level to be measured.
The coupling of the measurement signals from the transmitting unit to the conductive element and from the conductive element into the receiving unit occurs over a coupling unit, which is so structured that disturbance echo signals arising from impedance jumps in transition regions are kept as small as possible. Problematic in this is, among other things, that the coupling unit has a relatively small opening area. Due to the small opening area, on the one hand, an impedance jump occurs, which leads to an undesired reflection during emission of the measurement signals; on the other hand, the small opening area distorts the electric field, so that, besides exciting undesired oscillation modes, which exhibit a marked radiation characteristic in the direction of the fill material, also modes are produced, which have an undesired lateral radiation. As a result of the lateral radiation of the measurement signals, the signal/noise ratio can degrade considerably, depending on the details of the application. Besides this, following reflection of the measurement signal at the surface of the fill material, only that part of the returning wave which lies within the opening area of the coupling unit is coupled back into the transmitting-/receiving-unit.
In order to remove this undesirable situation and achieve an efficient exciting of the desired modes of the measurement signals on the conductive element, it is already known to use a horn-shaped element. This horn-shaped element is placed in the upper region of the conductive element and protrudes into the container in which the fill level is to be measured. Such a fill level measurement apparatus is described, for example, in DE 44 04 745 C2. The funnel-shaped element is supposed to care for optimizing the guiding of the measurement signal in the direction of the surface of the fill material. However, in using a funnel-shaped element, the radiation is only optimized when the wave length of the measurement signals is small relative to the diameter of the lower opening area of the horn-shaped element. If the wave length has the same order of magnitude as the diameter of the ham-shaped element, then no marked directional characteristic occurs in the travel direction of the measurement signals. Quite the contrary, a part of the energy travels back on the outer surface of the horn-shaped element, and gets reflected at the coupling unit, at the container wall, the flange or other obstacle, whereby relatively strong disturbing echos are caused. Consequently, in a large number of applications, the measurement results are actually degraded by the use of the horn-shaped element.